As everyone knows, functions related to a wireless local area network (WLAN), a global positioning system (GPS) or a global system for mobile communication (GSM) gradually become basic functions of a portable device. Thus, the portable device should have multiple built-in antennas to receive signals at different frequency bands. Since the size of the portable device is smaller and smaller, the size of the antenna is preferably smaller.
Generally, a chip antenna is the smallest antenna in size. To take a WLAN antenna as an example, the size of the chip antenna is less than 2 mm×5 mm×1 mm. However, the chip antenna is expensive, and its efficiency is not good.
There is another type of antenna which is a printed circuit board (PCB) antenna. The antenna is directly designed on the PCB. The PCB antenna has a low cost, but it occupies the largest area.
A wire antenna which is also called a monopole antenna may be designed on the PCB. The antenna includes a conducting line and a ground panel. The length of the conducting line is one quarter of the wavelength of the resonance frequency. Thus, the higher the resonance frequency is, the shorter the conducting line is. The lower the resonance frequency is, the longer the conducting line is. The conducting line is too long, and thus it is difficult to use the wire antenna in the mobile device directly.
To apply the wire antenna to the mobile device, generally the conducting line is designed to be winding type to reduce the area of the wire antenna. The winding conducting line may be a bent line or a serpentine line.
In addition, in pages 11 to 14 of the periodical, IEEE Antenna and wireless Propagation Letters 2007, a design of miniaturized printed wire antenna using double-layer periodic metallization is disclosed. In addition, in the periodical, IEEE APS 2008, a miniaturized printed wire antenna utilizing 3D substrate metallization for wireless communication is disclosed.
In the two periodicals, the wire antenna is designed utilizing the idea of an artificial transmission line. In the first periodical, a double-layer PCB is used, and the length of the WLAN antenna is reduced to about 12 millimeter (mm). In the second periodical, a three-layer PCB is used, and the length of the WLAN antenna is further reduced to about 8 mm. However, since the resonance frequency of both the two antennas cannot be tuned finely, the two antennas are hard to be used practically.